Dynamics of the East India Coastal Current: 1. Analytic solutions forced by interior Ekman pumping and local alongshore winds

TLDR

The study uses a linear, continuously stratified model to examine how interior Ekman pumping and local alongshore winds influence the East India Coastal Current. Analytic solutions of an approximate set of motion equations are derived for a Bay of Bengal–like basin forced by Hellerman and Rosenstein (1983) winds. The solutions reveal that interior Ekman pumping excites baroclinic Rossby waves that drive a northward coastal current when the interior circulation is anticyclonic, while local winds generate a surface‑mixed‑layer trapped current; the model reproduces the observed seasonal northward flow but lags its onset, fails to capture the Sri Lankan current reversal, and underestimates peak transport, indicating additional drivers are required.

Abstract

A linear, continuously stratified model is used to investigate how forcing by interior Ekman pumping and local alongshore winds affects the East India Coastal Current (EICC). Solutions are found analytically to an approximate version of the equations of motion. They are obtained in a basin that resembles the actual Bay of Bengal north of 6°N and are forced by Hellerman and Rosenstein [1983] winds. The mathematical structure of the solution clearly illustrates the model physics. In response to interior Ekman pumping, baroclinic Rossby waves are excited in the interior of the Bay; they propagate to the western boundary, where they generate a northward, coastal current if the interior circulation is anticyclonic and vice versa. In contrast, the response forced by the local alongshore winds is a coastal current that is trapped within the surface mixed layer. Consistent with the observed surface current, the model EICC flows northward along the Indian coast from March to September and equatorward along the Indian and Sri Lankan coasts from October to January. In contrast to the observations, however, the onset of northward flow along the Indian coast occurs 1–2 months late, the Sri Lankan coastal current does not reverse to flow southward during the summer, and the maximum northward transport of the model EICC is 5 Sv in May, only about half the transport estimated from hydrographic data. We conclude that, although interior Ekman pumping and local alongshore winds have a significant impact on the EICC, other driving mechanisms must be taken into account in order to simulate the observed variability adequately.